There are many plant based protein powders. Hemp, rice, pea, soy, etc. Are there any studies which compare the effectiveness of such powders in terms of muscle building? (For example hemp vs. rice powder)
I would recommend this article from Barbell Medicine, so you can decide for yourself while shopping for supplements. It is written by MD/coach/powerlifter and based on science and available data.
You'll need to eat more plant-based protein (if for regular diets people recommend 1g/1lb of body weight, you might need 1.5g/1lb)
As the quality of protein increases (based on bioavailability, protein digestibility amino acid corrected score, and amino acid profile) the total protein needed to optimize protein intake goes down. Similarly, the more vegan someone is, the more protein they require, i.e. the more calories from protein they require to get the same effect as their meat-eating, bone crushing, bacon frying counterparts.
You should look at leucine as primary amino acid, but there are caveats in terms of calories and quality of source (whey, meat, veggies)
Optimal protein intake per meal will be the amount of protein that yields ~3-4g of leucine, a branched-chain amino acid (BCAA). 3-4g of leucine per meal has been shown to maximize muscle protein synthesis.
In general, you should get most of your protein from non-supplements, i.e. from normal meals. (That's why they are called supplements). Think about why you want to take powder, what is missing in your body. Then start something that seems reasonable (product X, 1 scoop/day or whatever) and start tracking your progress. If you want to avoid gaining excessive amount of fat, waist circumference is a good indicator to track. If things going your way (lifts going up, biceps getting bigger, waist stays the same, whatever) continue as long as it works. And this article is good point of reference, I think.